1. Field of the Invention
The present invention relates to an improvement in a degree of crystalline of a lithium transition metal oxide thin film for an electrode of a thin film-type lithium battery (hereinafter referred as a lithium thin film battery), and more particularly to a method for crystallizing a lithium transition metal oxide thin film, in which the lithium transition metal oxide thin film is reacted with oxygen or argon plasma induced by a microwave (frequency: 2.45 GHz) or a radio frequency (hereinafter referred as an RF) wave (frequency: 13.56 MHz), thereby obtaining an excellent lithium transition metal oxide thin film in a degree of crystalline and electrochemical characteristics.
2. Description of the Related Art
According as rapid growth of portable electric equipments, such as; a notebook, a camcorder, a mobile telephone and a compact sound recorder has brought a increasing demand for these portable electric equipments, a development of batteries as an energy source for the equipments becomes an important problem to be solved, and there is a great rush for rechargeable secondary batteries among others. Particularly, a lithium secondary battery is being more investigated and commercialized than any other secondary battery.
Of these lithium secondary batteries, a lithium thin film battery is used in a field of microelectronics requiring charge/discharge capacity of 200 xcexcAh and lower electric power of 1 to 10 nA/cm2 and more specially in a field of MEMS (Micro Electro-Mechanical System), semiconductor memory, thin film-type gas sensor, smart card, micro medical equipment capable of being installed within the human body and so forth, and has an advantage that a shape and a size of the battery are adapted at one""s convenience to the application fields to be used.
A process for realizing the lithium thin film battery comprises the steps of: (1) preparing one type of substrate among a silicon substrate, an alumina substrate, a glass substrate and a metal substrate such as an aluminum substrate; (2) vapor-depositing current collectors for an anode and a cathode; (3) vapor-depositing an anode thin film; (4) vapor-depositing a solid electrolyte thin film; (5) vapor-depositing a cathode thin film; (6) vapor-depositing a protector film; and the like. Of these steps, vapor depositions of the anode, cathode and solid electrolyte thin films are the most important steps to determine an overall process temperature as well as duration and capacity of the battery. Lithium transition metal oxides, such as, LiCoO2, LiNiO2, LiMn2O4, V2O5, V6O13, SnO2, TiS2, LiAl, LiGaO2, LiTiO2, Li3PO4, etc. or mixtures thereof are used as material of these thin films. In order that these oxides may charge and discharge lithium ions in an electrochemically stable manner, LiCoO2 and LiNiO2 must be of a lamella structure, and LiMn2O4 must be of a spinel structure to have an excellent degree of crystalline. Of course, the other oxides show better duration and capacity of charge/discharge in a crystalline state than in an amorphous state.
At present, lithium transition metal oxides are thermally treated at a temperature between 750xc2x0 C. and 850xc2x0 C. for a time between several hours and several tens of hours in a furnace under an oxygen atmosphere and so crystallized before they are used in commercialized lithium secondary batteries. This furnace-thermal treatment performed at such a high temperature for such a long time gives rise to many problems in that (1) atoms are diffused to and mixed with each other between the anode, cathode and electrolyte thin films; (2) since metal substrates having a lower melting point cannot be used, there is no choice but to use materials having a higher melting point, such as, a ceramic substrate; and (3) the treatment cannot be used together with the existing semiconductor processes. As the result of that, it is impossible to apply the furnace-thermal treatment to the lithium thin film battery.
Therefore, many researchers have attempted to crystallize the lithium transition metal oxide thin film using a new technology. For example, P. Fragnaud et al., J. Power Source, 54 (1995), pp. 362-366 used a chemical gaseous vapor deposition, K. A. Striebel et al., J. Electrochem. Soc., 143 (1996), pp. 1821-1827 used a pulsed laser vapor deposition, and M. Yoshimura et al., Solid State Ionics 106 (1998), pp. 39-44 used a electrochemical-hydrothermal synthesis so as to seek to obtain a lithium transition metal oxide thin film having an excellent degree of crystalline directly at a stage of vapor deposition without a thermal treatment. In addition to these researches for improving the degree of crystalline directly at the stage of vapor deposition, there have been performed some researches for improving the degree of crystalline by a thermal treatment after a vapor deposition. For instance, Hong-Koo Baik et al., J. Electrochem. Soc., 143 (1996), pp. L268-L270 vapor-deposited a lithium transition metal oxide thin film by an electron beam evaporation and then applied a thermal treatment to the thin film in a furnace, and S. K. Joo et al., J. Electrochem. Soc., 141 (1994), pp. 3296-3299 vapor-deposited a lithium transition metal oxide thin film by sputtering and then performed rapid thermal annealing of the thin film, both researches being intended to improve the degree of crystalline of the lithium transition metal oxide thin film.
However, the attempts to improve the degree of crystalline directly at the stage of vapor deposition were not satisfactory, and although the attempts using the thermal treatment following the vapor deposition succeeded in the improvement of the degree of crystalline, they still have a shortcoming in lowering the process temperature to 500xc2x0 C. or less. Thus, there is a desire to develop a new treatment technology other than the existing technology.
Accordingly, the present invention has been made to overcome the above-mentioned problems, and it is an object of the present invention to provide a novel method for crystallizing a lithium transition metal oxide thin film, in which the lithium transition metal oxide thin film is treated at a lower temperature of 500xc2x0 C. or less for a shorter time without use of the conventional furnace-thermal treatment, thereby obtaining a lithium transition metal oxide thin film having an enhanced degree of crystalline and excellent electrochemical characteristics.
To achieve this object, there is provided a method for crystallizing a lithium transition metal oxide thin film in accordance with an aspect of the present invention, the method comprising the steps of:
vapor-depositing a lithium transition metal oxide thin film as electrode material on a substrate; and
treating the lithium transition metal oxide thin film by plasma.
Preferably, the lithium transition metal oxide thin film for an anode electrode of a lithium thin film battery is one type of oxide thin film selected from the group consisting of LiCoO2, LiNiO2, LiMn2O4, V2O5, V6O13, SnO2, TiS2, LiAl, LiGaO2, LiTiO2, Li3PO4 and mixtures thereof, and the plasma to be reacted with this oxide thin film is oxygen or inert gas plasma induced by a microwave (frequency: 2.45 GHz) or an RF wave (frequency: 13.56 MHz).
In another aspect of the present invention, a sputtering device used in the vapor deposition of the lithium transition metal oxide thin film according to the present invention is a reactive RF magnetron sputtering system device including a vacuum system, a sputtering target, an RF power source, a substrate holder and a gas injector.
It is preferred that the substrate is a Pt/Ti/SiO2/Si substrate in which p-type (100) silicon (Si) as a base is covered with silicon oxide (SiO2), titanium (Ti) and platinum (Pt) in sequence. The platinum acts as a collector, and the titanium serves to facilitate covering of the platinum on the SiO2 and to prevent diffusion of the platinum at a high temperature. Preferably, purity of the target used in the vapor deposition is 99.99% or more, argon and oxygen gases having a purity of 99.9999%, respectively are used as the reaction gas, and an initial degree of vacuum is 5xc3x9710xe2x88x927 Torr or less.
The vapor deposition of the lithium transition metal oxide thin film is preferably performed under the conditions that RF power is 100 W, a pressure of the vapor deposition is 3 mTorr, a temperature of the substrate is 350xc2x0 C., and respective flow rates of the argon and oxygen gases are 8 sccm. However, the present invention is not limited to these illustrative conditions of the vapor deposition, but the plasma treatment process according to the present invention is suitable to any lithium transition metal oxide thin films vapor-deposited under other conditions.
The essence of the present invention is in that the oxygen or argon plasma treatment of the lithium transition metal oxide thin film is performed as a post-treatment process following the vapor deposition of the thin film using the microwave or RF plasma treatment device, thereby enhancing the degree of crystalline, surface flatness and electrochemical stability of the lithium transition metal oxide thin film.
In still another aspect of the present invention, the microwave plasma treatment device is of a bell-jar-type using a microwave of 2.45 GHz, in which a graphite block having a size similar to that of the silicon substrate is installed on a substrate holder and a nonconductor quartz dome is used as a reaction tube. Within the reaction tube is provided a counter electrode comprising a stainless tube having a predetermined diameter and a kanthal wire wound around the stainless tube. Also, conditions of the oxygen plasma treatment are preferably such that microwave power is 500 W, a flow rate of the oxygen is 50 sccm, a pressure is 10 Torr or less, and a treatment time is 0 to 20 min, but the present invention is not limited to these conditions so that those skilled in the art can use any properly chosen conditions. On the other hand, the sputtering device is used as the RF plasma treatment device just as in the vapor deposition, but preferable conditions of the RF plasma treatment are such that an RF is 13.56 MHz, power is 20 W, a pressure is 3 mTorr, respective flow rates of the oxygen and argon gases are 8 sccm, and a treatment time is 0 to 20 min. Similarly, the present invention is not limited to these conditions and it is possible for those skilled in the art to use any properly chosen conditions.
Besides the oxygen and argon gases, inert gases, such as, helium, neon, krypton or xenon and mixture gas of oxygen and inert gas are available for the plasma treatment according to the present invention.
These gases are expected to give an ion etching effect and other advantageous effects due to chemical reactions. That is, such gases exist as ion/radical phases with high energy within the plasma, collide against the lithium transition metal oxide thin film to transmit thermal energy to the thin film, fill in cavities of the oxygen, and participate in a chemical bonding with inner metal atoms, thereby enhancing the degree of crystalline and flatness of the lithium transition metal oxide thin film.